Origin groups v5.6

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Rules for commit scopes can depend on the node the transaction is committed on, that is, the node that acts as the origin for the transaction. The bottom group of the group tree to which that node belongs is the transaction's origin group. To make this transparent for the application, PGD allows a commit scope to define different rules depending on the transaction's origin group.

For example, consider an EDB Postgres Distributed cluster with nodes spread across two data centers: a left (left_dc) and a right one (right_dc). Assume the top-level PGD node group is called top_group. You can use the following commands to set up subgroups and create a commit scope requiring all nodes in the local data center to confirm the transaction but only one node from the remote one:

-- create sub-groups
SELECT bdr.create_node_group(
    node_group_name := 'left_dc',
    parent_group_name := 'top_group',
    join_node_group := false
);
SELECT bdr.create_node_group(
    node_group_name := 'right_dc',
    parent_group_name := 'top_group',
    join_node_group := false
);

-- create a commit scope with individual rules
-- for each sub-group
SELECT bdr.add_commit_scope(
    commit_scope_name := 'example_scope',
    origin_node_group := 'left_dc',
    rule := 'ALL (left_dc) GROUP COMMIT (commit_decision=raft) AND ANY 1 (right_dc) GROUP COMMIT',
    wait_for_ready := true
);
SELECT bdr.add_commit_scope(
    commit_scope_name := 'example_scope',
    origin_node_group := 'right_dc',
    rule := 'ANY 1 (left_dc) GROUP COMMIT AND ALL (right_dc) GROUP COMMIT (commit_decision=raft)',
    wait_for_ready := true
);

Now, using the example_scope on any node that's part of left_dc uses the first scope. Using the same scope on a node that's part of right_dc uses the second scope. By combining the left_dc and right_dc origin rules under one commit scope name, an application can simply use example_scope on either data center and get the appropriate behavior for that data center.

Each group can also have a default commit scope specified using the bdr.alter_node_group_option admin interface.

Making the above scopes the default ones for all transactions originating on nodes in those groups looks like this:

SELECT bdr.alter_node_group_option(
  node_group_name := 'left_dc',
  config_key := 'default_commit_scope',
  config_value := 'example_scope'
);
SELECT bdr.alter_node_group_option(
  node_group_name := 'right_dc',
  config_key := 'default_commit_scope',
  config_value := 'example_scope'
);

ORIGIN_GROUP

You can also refer to the origin group of a transaction dynamically when creating a commit scope rule by using ORIGIN_GROUP.

This can make certain commit scopes rules like those above in example_scope, even easier to specify in that you can simply specify one rule instead of two.

For example, again suppose that for transactions originating from nodes in right_dc you want all nodes in right_dc to confirm and any 1 from left_dc to confirm before the transaction is committed. Also, again suppose that for transactions originating in left_dc you want all nodes in left_dc and any 1 in right_dc to confirm before the transaction is commited. Above we used these two rules for this when defining example_scope:

SELECT bdr.add_commit_scope(
    commit_scope_name := 'example_scope',
    origin_node_group := 'left_dc',
    rule := 'ALL (left_dc) GROUP COMMIT (commit_decision=raft) AND ANY 1 (right_dc) GROUP COMMIT',
    wait_for_ready := true
);
SELECT bdr.add_commit_scope(
    commit_scope_name := 'example_scope',
    origin_node_group := 'right_dc',
    rule := 'ANY 1 (left_dc) GROUP COMMIT AND ALL (right_dc) GROUP COMMIT (commit_decision=raft)',
    wait_for_ready := true
);

However, with ORIGIN_GROUP, just adding and using the following single-rule commit scope, example_scope_2, will have the same effect as the two individual rules we used above in example_scope:

SELECT bdr.add_commit_scope(
    commit_scope_name := 'example_scope_2',
    origin_node_group := 'top_group',
    rule := 'ALL ORIGIN_GROUP GROUP COMMIT (commit_decision=raft) AND ANY 1 NOT ORIGIN_GROUP GROUP COMMIT';
    wait_for_ready := true
);

Under example_scope_2, when a transaction originates from left_dc, ORIGIN_GROUP maps to left_dc and NOT ORIGIN_GROUP maps to right_dc. Likewise, when a transaction originates from right_dc, ORIGIN_GROUP maps to right_dc and NOT ORIGIN_GROUP maps to left_dc. So by only specifying one rule, you get the effect of two.

Note that if you added more subgroups, for instance a third child of top_group, middle_dc, then according to example_scope_2 above, for transactions originating from left_dc, all the nodes in left_dc must plus any 1 in right_dc and any 1 in middle_dc must confirm before the transaction is committed. Of course then for transactions originating in right_dc all the nodes in right_dc plus any 1 node in left_dc and any 1 node in middle_dc must confirm before the transaction is committed. Lastly, because middle_dc is a child of top_group, example_scope_2 also means that for transactions originating in middle_dc, all the nodes in middle_dc plus any 1 node in left_dc and any 1 node in right_dc must confirm before the transaction is committed.

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